1. Field of the Invention
The present invention relates to a reflection type liquid crystal display device. More specifically, the present invention relates to a reflection type liquid crystal display device having a front light.
2. Description of the Related Art
Reflection type liquid crystal display devices are suitable for low power consumption since their back light need not be illuminated at all times. They are employed for cellular phones and portable information terminal devices so called PDA (Personal Digital Assistant).
The reflection type liquid crystal display devices use mainly a light from an outside light for display. In many cases, they have an auxiliary light source called a front light by assuming their use under an environment in which an outside light cannot be expected. The basic construction of a reflection type liquid crystal display device with a front light will be described below.
FIG. 10 and FIG. 11 show the constructions of a reflection type liquid crystal panel and a front light of a prior art. A reflection type liquid crystal panel 17 has a liquid crystal layer 173 interposed between a pixel substrate 171 having a reflection pixel electrode and a color filter substrate 172.
As shown in FIG. 10, a front light 16 is arranged at the frontward side (observer's side) of the reflection type liquid crystal panel 17, and has a function of allowing a light from a light source part 18 to be incident onto the reflection type liquid crystal panel 17 to pass the light reflected in the inner part of the reflection type liquid crystal panel 17 through the observer's side.
As shown in FIG. 11, the reflection type liquid crystal panel 17 has a reflection electrode 1 of uneven structure for reflecting a light onto a TFT substrate, and forms one pixel 19 by three display devices 2 corresponding to red, green, and blue colors (R, G, and B) for color display. A method for forming a random uneven pattern in one display device 2 or one pixel 19 as a basic unit is typical.
FIG. 12 is a diagram showing change in brightness of an emitted light from the front light 16 to observation angles. As seen from the drawing, to obtain bright display at illuminating the front light, the light is emitted in a direction almost vertical to the reflection type liquid crystal panel 17.
FIG. 13 shows the characteristic of a reflector of the prior art. It shows light-receptive angle dependence of reflectivity to a parallel incident light having an incident angle of 30°.
As shown in FIG. 13(a), the reflection type liquid crystal display device is often used outdoors. In this case, an outside light such as a sunlight or fluorescent lamp is used to see display of the liquid crystal panel.
The brightness is highest at the regular reflection position of the outside light. Since the image of the outside light is observed, the display of the liquid crystal panel is hard to see. The reflector of the prior art has been designed so as to obtain bright display at an angle shifted several tens of degrees from the regular reflection position (30°) of the outside light, as indicated by the arrow in FIG. 13(a).
In the reflection type liquid crystal panel having a front light, as shown in FIG. 13(b), such a reflector is proposed to emphasize the effective use of a light from the front light.
A white-color light-emitting diode is often employed as a substitute for a cold-cathode tube in view of low power consumption and low noise. Since the light-emitting diode has a smaller input power compared to that of the cold-cathode tube, it is hard to obtain a brightness higher than that of the cold-cathode tube.
Specifically, to obtain high brightness at illuminating the front light, the flat part of the reflector is increased to make a regular reflection component larger.
As a result, as shown in FIG. 13(b), bright display can be obtained at an angle near the regular reflection.
The prior art reflection type liquid crystal device having a front light, however, has a reflective characteristic in which a viewing angle is narrow although high brightness can be obtained at illuminating the front light. A bright display cannot be obtained with outside light when not illuminating the front light.
Light usage, in an optical system, is different for outside light than for a front light. Thus, a prior art reflection type liquid crystal device having a front light has difficulty obtaining a bright display under both the environments.